1. Field of the Invention
The present invention concerns frame synchronization in a binary data transmission system in which the data is transmitted in the form of an isochronous bit stream organized into frame provided with an alignment word formed of groups of bits which may be reduced to a single element distributed through the frame.
2. Description of the Prior Art
A transmission system of this kind is used at subscriber line level in an Integrated Services Digital Network (ISDN). In this context, the subscriber terminals are placed on standby in the absence of any communication and activated at the start of each communication. Their activation, which must be as fast as possible, requires the shortest possible frame synchronization time.
Searching for an alignment word distributed through a frame is more often than not implemented with the aid of a shift register storing the data received over the duration of an alignment word with parallel outputs arranged relative to one another so as to cause to appear at certain times all the bits of an alignment word and a decoder connected to these parallel outputs of the shift register and identifying the alignement word each time that it appears.
This kind of search, which is very fast, lasting at most slightly less than the duration of two consecutive frames, often requires a shift register with a very large number of stages as it is not rare for an alignment word to extend over more than 100 bits of a frame.
To avoid this disadvantage it has been proposed to search for the alignment word by means of a series of tests consisting in searching for the first bit received compatible with the value of the first bit of the alignment word, adopting the frame subdivision corresponding to it and verifying one by one the following bits of the alignement word determined by this subdivision, any negative test entailing resumption of the synchronization procedure from the beginning. This process has the advantage of no longer requiring the memorization of all data bits received over the duration of an alignment word, but the disadvantage of being slow.
An object of the present invention is a frame synchronization process which is simple to implement and faster than that just described.